In the majority of devices and applications requiring electrical contact, the required contact occurs at an essentially planar (2D) interface between an electrode and the material being contacted. What appears planar at long length scales generally acquires some corrugation at small length scales. However this corrugation is generally a natural consequence of the materials rather than a feature specifically engineered into the interface. However, numerous applications can benefit from an electrical contact that is 3-dimensionally distributed.
Examples of applications that can benefit from 3-dimensionally distributed contacts include electrodes for electrochemical reactions such as for the production of hydrogen from water and proton generation at the anode in hydrogen fuel cells. In such applications the increased surface area electrode provides an increase in electrochemically generated product. For super-capacitors, the increased electrode surface area greatly increases the device capacitance. Other applications, such as for solar cells or photodetectors, where light must be absorbed within a semiconducting junction region possessing a built-in potential to drive the photo-generated electrons to the cathode, can similarly benefit from the extended active area volume that a 3-dimensionally distributed electrode can provide. For electroluminescent device applications increased surface area electrical contact to the active material can provide increased current injection, with concomitant increases in light generation.
Recently, films of single-wall carbon nanotubes (SWNTs), which are electrically conducting have emerged as promising electrodes for a broad range of applications. Such films can be fabricated by various methods including a method described in published U.S. Application No. 20040197546 (hereafter the '546 application) to a group of inventors including one of the present Inventors. The '546 application is incorporated by reference into the present application in its entirety. Briefly, the method described in the '546 application comprises filtration of a surfactant suspension of SWNTs onto the surface of a filtration membrane possessing pores too small for the SWNTs to pass through. The nanotubes accumulate at the surface of the membrane forming a film. Subsequent washing removes residual surfactant, while drying consolidates the nanotube film. Transfer of the film to a substrate of choice requires appropriate selection of the membrane media to permit its dissolution in a solvent that can be tolerated by the substrate to which the transfer is made. Such transfer generally proceeds by adhering the membrane-backed nanotube film to the substrate, followed by dissolution of the membrane in the chosen solvent.
SWNT films so fabricated possess a tortuous path, open porosity in which the pores between nanotubes are defined by the overlapping and crossing nanotube bundles. The nanotubes tend to be self-organized into bundles, each possessing a varying number of nanotubes across their widths from a few to hundreds of parallel nanotubes, approximately 3 to 20 nm in diameter, with a typical diameter of ˜10 nm. FIG. 1 shows a scanned atomic force microscopy (AFM) image of a typical 70 nm thick film surface (bundles diameters appear greater than ˜10 nm only because of tip-sample convolution). This open porosity has the potential to provide a structure having some of the desired, high surface area, 3-dimensionally distributed electrical contact with another material.
Examination of FIG. 1 suggests that voids between crossing nanotube bundles have dimensions of tens to hundreds of nanometers across. However, the inference of pore volumes from such surface images is misleading. In the film formation process disclosed in the '546 application the nanotube bundles are uniformly dispersed in the dilute, aqueous suspension. The first bundles to land on the flat filtration membrane surface are forced to lie essentially parallel to the surface. Because the film grows at a uniform rate (with nanotube bundles lying across those deposited before them), subsequently deposited bundles take on the same planar orientations. The result is a film morphology wherein the nanotubes have random in plane orientations, but lie in stacked planes, with two-dimensional anisotropy similar to a biaxial oriented polymer film. This would suggest that the average dimension of the pores between bundles, in the direction perpendicular to the filtration membrane surface (the thickness direction of the film), is that of only a few nanotube bundle diameters. This analysis assumes however that the nanotube bundles are rigid rods.
The nanotube flexibility, and surface energy minimization by van der Waals contact causes them to maximize their contact, acting to further reduce these pore volumes. A quantitative measure of the available porosity is given by a comparison of the theoretical density of a hexagonal close pack array of nanotubes (using a prototypical 1.356 nm diameter (10, 10) nanotube) and the experimentally derived density of a filtration method formed SWNT film. The former is approximately 1.33 g/cm3 while the latter has been measured to be about 0.71 g/cm3. Hence the as-produced filtration method described in the '546 application produces SWNT films that achieve nearly 53% of their theoretical maximum density. Since this porosity is generally uniformly distributed throughout the film, the average pore volume is generally of a size that is smaller even than an average bundle volume.
There may be utility to infiltrating the porosity of films produced using the process disclosed in the '546 application with an electro-active medium and using the nanotubes as electrodes. However, the small size of these pores limits the utility of this structure for 3-dimensional distributed electrode applications. The limitations associated with the small pore size depends on the specific application, two exemplary limitations being as follows:
1. As electrochemical electrodes the small pores yield slow dynamics for permeating chemical species into the volume of the films, against the countercurrent of reaction products that must get out. This will limit the production rate of the desired species.
2. As photovoltaic electrodes, which are infiltrated with a semiconductor that generates a built-in potential at the nanotube-semiconductor interface, wherever the nanotubes defining the pores lie within a Debye length proximity of each other, their potentials will screen each other, reducing the potential gradient. Since that potential gradient provides the electromotive force for charge transport away from the interface, such screening will limit the photo-current and therefore the power generated by the photovoltaic device.
Thus, a need exists for nanotube and/or nanowire films having higher levels of porosity, significantly higher pore volumes, and a higher ratio of surface area to film volume as compared to films produced by the method described in the '546 application, or other methods of nanotube film fabrication such as spray coating or Langmuir-Blodgett assembly.